1. Field of The Invention
The present invention relates to an apparatus and method for optimally controlling damping force characteristics of four tire wheel suspension units (shock absorbers) of an automotive vehicle.
2. Description of Background Art
A Japanese Patent Application First Publication No. Heisei 4-191111 published on Jul. 9, 1992 exemplifies a previously proposed damping force characteristic control apparatus for respective shock absorbers interposed between a corresponding vehicular body (sprung mass) and tire wheel assemblies (unsprung mass).
In the above-identified Japanese Patent Application Publication, a plurality of actuators (shock absorbers) each interposed between the vehicle body and a rear tire (road) wheel so as to enable an increase or decrease in a suspension force (damping force characteristic) on the vehicle body with respect to the corresponding rear tire (road) wheel, a vertical acceleration sensor arranged so as to detect a vertical acceleration acted upon the vehicle body due to a vibration input from front tire wheels caused by a recess and convex formed on a road surface on which the vehicle is running, a vehicle speed sensor arranged so as to detect the vehicle speed, and control unit arranged so as to control operations of the actuators on the basis of input signals from the above-described sensors are provided in the vehicular suspension control apparatus.
The control unit described above is so constructed as to actuate the actuators, when determining that the vehicular body vertical acceleration detected from each of the vertical acceleration sensors exceeds a predetermined value, calculates a delay duration in time for which the rear road wheels have reached to the corresponding recess and convex of the road surface which has given the sprung mass vertical acceleration which exceeds the predetermined value and actuates the actuators described above according to the magnitude of the control signal derived by inverting a signal based on the vertical sprung mass acceleration after the calculated delay duration in time has passed upon passage of the front tire (road) wheels on the recess and convex of the road surface.
That is to say, in the above-identified Japanese Patent Application First Publication, even when a relatively large vibration occurs on the vehicle body at the time when the front road tire wheels have passed such a recess and convex of the road surface as described above, the control for the rear road wheel side shock absorbers is carried out so as to cancel the vertical acceleration acted upon the vehicle body by referring to the vertical acceleration acted upon the vehicle body derived at the time when the front road wheels have passed on the recess and convex of the road surface, when the rear road wheels pass on the same recess and convex of the road surface. Consequently, when the rear road wheels have passed on the same recess and convex on the road surface, the vibration input derived at the time when the front road wheels have passed the same recess and convex of the road surface can be reduced.
However, the following problems occur in the above-described shock absorber damping force characteristic control apparatus.
In general, the vertical acceleration indicative signal detected by means of the vertical sprung mass G (gravity) sensor is converted into a sprung mass vertical velocity by means of an integrating method using a low pass filter or undergoes a variety of filtering processing to reduce or eliminate noise components or unnecessary components. In this way, when a signal processing (filtering) is carried out to derive a frequency-dependent characteristic signal as described above, a phase of a processed signal tends to become an advanced (lead) state with respect to the phase of the sprung mass vertical velocity when the processed signal has a relatively low frequency and tends to become a retarded (lag) state when the processed signal has a relatively high frequency (this is appreciated from a dot line depicted in FIG. 15). Therefore, especially, when the phase is delayed due to the case when the high frequency input exceeding a sprung mass resonance frequency to the vehicle body occurs, the desired control signal cannot be obtained and, correspondingly, a vehicular riding comfort can be reduced.
It is noted that, in the above-described vehicular shock absorber control apparatus, a preview control such that a timing at which the control signal is used to control the damping forces of the shock absorbers located at the rear road wheel positions is retarded according to a vehicle speed is carried out and no elimination of the phase deviation of the filter processed signal is carried out.
Furthermore, as shown in FIG. 1, when an impulse road surface input occurs at the front road wheels (when the front road wheels pass a road surface having a recess and convex) , an interference between vehicular body front and rear portions causes simultaneous starts of behaviors at the front road wheel sides (front road wheel side sprung mass vertical velocity) and those at the rear road wheel sides (rear road wheel side sprung mass vertical velocity), in the case of the actual behaviors of the vehicle. Thus, as described above, since the preview control such that the timing at which the control signal derived at the front road wheel sides is used for the control at the rear road wheel sides is delayed according to the magnitude of the vehicle speed, a lag in a control force applied to those at the rear road wheel sides always occurs. In addition, since the control force at the rear road wheel sides is changed during the start of behaviors due to the road surface input at the front road wheel sides. Consequently, a discrepancy of feeling, in turn, occurs.